Field of the Invention
This invention relates to a system and method for tracking a feature on an object in real time without path preprogramming and, more particularly, to a seam tracking system incorporating one or more kinematically redundant axes, where the system is used for welding or sealant or adhesive application.
Description of the Prior Art
Robotic seam tracking is known and the seam or feature to be tracked or followed is identified using a sensor attached to an end or tool of the robot. The sensor monitors the surface of the workpiece in which the seam or feature is located, slightly ahead of the tool. The end of the robot is moved along the seam or feature while maintaining a desired orientation of the tool with respect to the surface. Previous arc welding systems are often programmed on-line, point by point, along the seam to be welded. This can be a very time consuming procedure and inaccurate positioning or jigging of the workpiece or poor joint preparation can result in the production of unacceptable welds. The accuracy of the previous systems can be improved by improving the dimensional accuracy of the components to be welded and improving the consistency with which these components can be jigged or, alternatively, by attempting to compensate for part-to-part variation in both dimensions and fixturing with the use of on-line sensing of the location and preparation of the seam. In low volume manufacturing or in applications where weld quality is of the utmost importance, the use of seam sensing is the more viable approach since this enables the controllable welding parameters to be adjusted in response to sensed joint dimensions and location.
In previous robotic arc-welding systems, a seam tracking sensor is located ahead of the welding torch to determine the location of the seam to be welded. This usually requires a nominal torch trajectory to be pre-programmed, based on the shape of the surface being tracked. The sensor can also used to measure joint dimensions so that torch travel speed and offset may be adjusted to compensate for variations in fit up and joint preparation. This requires that the seam be mapped in such a way that these parameters can be changed over the time delay between obtaining the data and welding the seam at that location.
When tracking a feature or seam on a stationary workpiece, the location of any point on the seam, as sensed by the seam tracker, may be expressed in absolute coordinates (i.e. in a coordinate frame that is fixed with respect to the robot base). This is possible since the position of the end-point is known in absolute coordinates (through the forward kinematics of the robot), the location of the seam tracker coordinate frame is known with respect to the end-point, and the seam feature location is defined in the seam tracker coordinate frame. Since the workpiece is stationary, it would be equally valid to express the sensed seam location in a coordinate frame attached to the workpiece, this workpiece frame being related to the absolute coordinate frame by a fixed transformation, which is denoted as T.
Further, it is known to use a system for real-time seam tracking for a welding robot without preprogramming as described in a thesis presented at the University of Waterloo, Waterloo, Ontario, Canada by D. L. Strauss in 1991 entitled "Real-Time Seam Tracking and Torch Control For a Welding Robot".
Previous seam tracking systems can suffer from disadvantages in that they require path preprogramming, which can be very time consuming and expensive. If the workpiece is not required to move relative to a robot base, this can be reasonably accommodated with the current practice. If the workpiece is required to continuously move relative to the robot base, a workpiece positioner is used; to provide control of the relative end-point to surface velocity, a large number of accurately sequenced locations must be preprogrammed for both the positioner and the robot corresponding to the coordinated motion required.